Assessment of the dynamics of terrestrial vegetation using satellite observations of greenness and sun-induced chlorophyll fluorescence

Title:

Assessment of the dynamics of terrestrial vegetation using satellite observations of greenness and sun-induced chlorophyll fluorescence

Author(s):

Walther, Sophia

Issue Date:

2019

Available Date:

2019-01-31T14:46:24Z

Abstract:

Photosynthesis is one of the most fundamental processes on Earth fuelling life
by providing food and energy. Moreover, terrestrial vegetation is a key element in the
climate system as it importantly affects exchange processes of carbon, water and energy
between the land surface and the atmosphere. In times of a changing climate there is
urgent need for detailed knowledge on the factors driving plant activity and for reliable
observational systems of the terrestrial vegetation. Satellite remote sensing is the only
means to obtain measurements with global coverage, including remote and inaccessible
regions, in a spatially and temporally continuous manner. This thesis presents an assess-
ment of our current observational capabilities of vegetation dynamics from space. Three
complementary approaches of spaceborne ecosystem monitoring are inter-compared: 1)
Spectral measurements of the land surface reflectance in the optical range give an indica-
tion of the amount of green biomass (as an integrative signal of leaf quantity and quality)
and hence of the potential to perform photosynthesis. 2) In the red and far-red spectral
regions, satellite instruments register a very small additive signal to the reflected radiance
which originates from photosynthetically active chlorophyll pigments, termed sun-induced
chlorophyll fluorescence (SIF). 3) Carbon fluxes measured in-situ are upscaled to a global
data set of model gross photosynthetic carbon uptake (known as GPP - gross primary
production) using empirical relationships with remotely sensed land surface and environ-
mental variables. Three case studies focus i) on the spring phenology in boreal forests,
ii) on the peak growing season in circumpolar treeless regions, and iii) on phenological
changes in ecosystems with varying abundances of trees globally in times of fluctuations
in soil moisture availability. The results of all three case studies highlight the intrinsic
differences between greenness on the one hand and photosynthetic activity on the other
hand. Specifically – for the first time on synoptic scales – a decoupling of photosynthesis
(as indicated by SIF and model GPP) and greenness (approximated by various indices
derived from spectral reflectance measurements) could be observed in evergreen needleleaf
forests during spring recovery. Similarly, a temporal mismatch occurs in northern hemi-
sphere forests during the growing season. There, changes in incoming light co-vary with
soil moisture and immediately affect photosynthetic performance but barely greenness.
Moreover, it has emerged that the timing of peak photosynthesis and peak greenness are
asynchronous in tundra areas, which is indicative of differing dynamics. Conversely, there
is high consistency between the three approaches regarding the length of growing season
in deciduous forests and moisture-related phenological shifts in non-forested ecosystems.
The work in this thesis demonstrates that SIF represents an asset for the monitoring
of the dynamics of photosynthesis and carbon uptake compared to greenness-based ap-
proaches. There are further indications of SIF to track changes in photosynthetic yields.
However, despite these promising results for the accurate tracking of photosynthesis from
space, further research is required to provide higher resolution data sets with clearer sig-
nals. Further, ground-based validation efforts are necessary to improve our mechanistic
understanding of physiological and radiative transfer processes controlling the SIF signal.